The purpose of this research study is to find out whether a new drug, rosiglitazone, when combined with two other anti-diabetic drugs called glyburide and metformin, is effective in treating patients who have type 2 diabetes mellitus. Glyburide and metformin are medications already approved for use by the FDA. These drugs are believed to work in different ways in controlling diabetes. The purpose of this study is to find out if rosiglitazone plus glyburide and metformin, along with diet, works to improve blood sugar control better than glyburide plus metformin and diet alone. The safety of rosiglitazone will also be studied when taken with both glyburide and metformin.